Closed circuit multi-stage air classifier

ABSTRACT

A closed-circuit air classifier for comminuted and particulate materials, especially wood fibers, sawdust, pulverized-wood particles and the like, which comprises a coarse air classifier and a fine classifier, each of which is provided with respective fan or whizzer blades for suspending or entraining the particles with respective air streams. The inlet of the coarse air classifier is connected to the outlet of the fine air classifier and is provided with a pneumatic gate through which the material to be classified is introduced into the system. The outlet of the coarse classifier is connected to the inlet of the fine classifier.

United States Patent 1 Voelskow 1 Oct. 23, 1973 [75] lnventor: Peter Voelskow, Bad Kreuznach,

Germany [73] Assignee: G. Siempelkemp & C0., Krefeld,

Germany [22] Filed: Mar. 17, I971 [21] Appl. No.: 125,140

[30] Foreign Application Priority Data Prouty 209/144 X 2,018,669 10/1935 Hermann 55/340 X 2,269,412 1/1942 Sturtevant.... 209/144 2,460,938 2/1949 Koehne 209/144 X r 3,615,009 10/1971 Norton 209/144 X FOREIGN PATENTS OR APPLICATIONS 156,869 9/1939 Austria 209/144 1,200,996 8/1970 Great Britain 209/148 Primary Examiner-Frank W. Lutter Assistant Examiner-Ralph J. Hill Attorney-Karl F. Ross [57] ABSTRACT A closed-circuit air classifier for comminuted and particulate materials, especially wood fibers, sawdust, pulverized-wood particles and the like, which comprises a coarse air classifier and a fine classifier, each of which is provided with respective fan or whizzer blades for suspending or entraining the particles with respective air streams. The inlet of the coarse air classifier is connected to the outlet of the fine air classifier and is provided with a pneumatic gate through which the material to be classified is introduced into the system. The outlet of the coarse classifier is connected to the inlet of the fine classifier.

12 Claims, 5 Drawing Figures PAIENTEB 0U 23 I13 SHEEI 10F 4 Peter Voelskow Inventor.

Attorney PATENIEDUCI 23 ms SHEET 2 [IF 4 PATENIEBMIN I913 3. 767,045 SHEET 3 OF 4 3 Pefer Voelskow Invenfor.

y WA 3 Attorney FIG. 4

Pefer Voelskow Inventor.

By s

Attorney CLOSED CIRCUIT MULTI-STAGE AIR CLASSIFIER FIELD OF THE INVENTION The present invention relates to air classifiers and, more particularly, to an air classifier of the type in which a stream of gas, generally air, entrains a particulate material to permit relatively coarse particles to sediment or deposit under gravitational and/or centrifugal force at one collection point while the lowergravity particles are carried by the gas to another collection point; more particularly, the invention relates to a closed-circuit air classifier enabling the collection of relatively heavy and relatively light particles of comminuted wood, e.g. for the preparation of pressed board.

BACKGROUND OF THE INVENTION In the manufacture of pressed board, wood or other fibers, wood dust, wood chips and, in general, particles of comminuted wood are pressed together, generally in the presence of heat, with or without extrinsic binders to produce a compact structure which may be finished or left unfinished, which may be used as is, or as a starting material for laminated structures, and which may have varying degrees of insulating ability, compressive strength and porosity.

It is a common practice in the manufacture of such pressed board, Le. hard board, low-density board, laminate cores and insulating or structural panels, to press loosely coherent or noncoherent mats of wood particles under heat and pressure in platen presses. The density and other physical properties of the product will depend, of course, on the amount of binder used, e.g. the quantity of binder added or the quantity and nature of the binder naturally present in the wood particles, upon the press pressure and temperature and, of course, upon the nature of the particles. For example, the pressed board may have a layer of fine particles disposed along the opposite sides of a layer of coarse particles to improve the texture, moisture resistance and associated properties of the finished product. A plant for the production of pressed board may, therefore, include dispensing devices for wood particles of various sizes, comminution apparatus, (e.g. mills for producing wood particles) and classifying apparatus for separating the milled wood products into particles of the desired size.

Various separating systems have been provided for classifying particles, in the pressed-board industry and elsewhere. For example, it is possible to separate particulate materials by their size using sieves or by their mass using gravitational phenomena. For example, cyclones may be used to remove particles of a particular size from a stream of air or other fluid in which the particles are entrained. Other separating systems make use of hydraulic, sedimentation and analogous techniques.

Of principal interest in the field of pressed-board manufacture, however, are air classifiers which operate under gravitational principles, namely, the fact that particles entrained in a gas stream will settle therefrom at rates depending upon their mass. While Newtonian physics predicts that the rate of fall of a body is independent of its mass, in practice the mass/volume/surface-area characteristics of a particle in a moving air stream determined the rate of fall and these principles, developed by Stokes and others, have long been uti-; lized to separate large particles from small particles.

The operating characteristics of air classifiers are discussed in chapter 8, pages 27 ff. of PERRY's CHEMI- CAL ENGINEERING HANDBOOK, 4th Edition, McGraw-Hill Book Company New York, 1963. cyclone/filter Pneumatic classifiers in which wood particles are permitted to descent through a rising air stream, have been used in the pressed-board industry for some time (see the November 1969 issue of the publication HOLZ ALS ROH-UND WERKSTOFF": Die Modeme Spanplatten Fertiqung by Max Himmelheber and Walter Kull, page 405). Such air classifiers, however, require long columns, high-power blowers to generate the rising stream of air, and at least one cyclone-type separator to remove fine particles from the air before it is released in the atmosphere. With rising concern about air pollution, however, even the results of the cyclone are unsatisfactory because of residual particles in the air entering the atmosphere. At least one filter is therefore required as well. The cost of such systems is relatively high and the operating costs always prohibitive. More specifically, it has been found that the blower dimensioned are governed by the high flow resistance of the cyclon/filter combination and the added blower cost is accompanied by higher power requirements. However, the principal disadvantage of such systems is that the release of the classifying air into the atmosphere causes emission of particles which aretoo small to be recovered in the cyclone or trapped upon filters of commercially satisfactory pore size. Of course, it is possible to use cloth or other filters with smaller pore size, but this is accompanied by an increased pressure drop, the constant need for cleaning or replacement of the filter material and the difficulty of operating for long periods without cleaning of the filter.

It has also been proposed to avoid the prior disadvantages by recirculating the classifying air at the desirable side of the cyclone and thereby avoid releasing the classifying air into the atmosphere. These systems have been found to be impractical because of peculiarities of the cyclone and the need for complicated conduit and piping arrangements which create additional problems.

Mention may also be made of so-called centrifugal air classifiers which enable the air-classification chamher to be reduced to a much smaller volume because of the higher air velocity and the lower losses in transferring energy from the fan or whizzer blades to the particles. However, even these systems have required cyclones, filters and the like at the output side to prevent contaminating the atmosphere.

OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved air-classification apparatus which operates without atmospheric contamination and affords, with low operating and capital costs, an efficient size separation of particulate material. 7

It is another object of the invention to provide a-convenient, efficient and low-cost apparatus for separating wood particles into coarse and fine fractions.

Still another object, of equal importance, is to provide an apparatus for the separation of a solid mix into particles of various sizes which has small spatial requirements, is of relatively inexpensive manufacture and most importantly does not contaminate the atmosphere.

It is another object of the invention to provide a convenient, efficient and low-cost apparatus for separating wood particles into coarse and fine fractions.

Still another object, of equal importance, is to provide an apparatus for the separation of a solid mix into particles of various sizes which has small spatial requirements, is of relatively inexpensive manufacture and most importantly does not contaminate the atmosphere.

SUMMARY OF THE INVENTION These objects and others will become apparent hereinafter are attained, in accordance with the present invention, in an air pneumatic classifier which comprises two air classifiers or separators connected in series and forming a closed circuit, including at least one coarse classifier and at least one fine classifier respectively having discharge gates through which the coarse and fine fractions can respectively be removed.

According to the principles of the present invention, the outlet of the discharge side of the coarse classifier is connected to the intake side or inlet of the fine classifier and the discharge or outlet side of the latter is connected to the intake or inlet side of the coarse classifier by suitable conduits forming the closed circuit.

Each of the classifiers according to the present invention comprises a blower chamber and a classification chamber in succession and provided with air or gas displacement members, preferably fans, adapted to drive the classification air or gas through the seriesconnected classifier chamber or to induce (by generating negative or low pressures) the flow of classifying air through the system. The cycle is closed off from the atmosphere but at least along some point of the circulation path, a gate is provided to admit the solids to be separated.

According to the invention, the rotors of the two units may be driven in the same sense in which case the conduit means may form with the classification units, an oval cycle; alternatively, the rotors of the units are driven in opposite senses and it has been found to be advantageous, in this case, to connect the conduits to these units in a figure-8 pattern. In general, the fine separator will be provided with a rotor which is longer and of larger diameter than the coarse-separator rotor and the classifying-air stream is directed at the coarse separator rotor with a higher radial speed than that of the gases directed at the fine-separator rotor. The particles in the coarse separator are subjected to a lesser tangential acceleration, thereby permitting the larger particles to deposit from the air stream while the lighter particles are entrained with the radially inflowing air.

It has been found to be possible to eliminate a gate at the inlet to the closed path when path of the classifying air is vented from the system, generally at one or both of the discharge gates and most commonly through a cell gate blocking the escape of particles. This gate can be in the form of a valve alternately open to the atmosphere and the closed circuit. However, it is prefered to provide gates at all three openings to the atmosphere and, when three such gates are not desirable, to provide at least two gates with only one of the openings being free from a gate maintaining the pressure within the closed system.

According to still another feature of the invention, the coarse separator is provided, in its classification chamber, with guide elements for the air stream entering the chamber and orienting this stream at an angle to the radial and axial directions against the coarseparticle rotor, i.e. in a generally spiral direction. While it is possible within the present framework to direct the classifying airstream tangentially against the coarseseparator rotor, it has been found that the former technique is most desirable. In either case, the light particles are entrained in a circular-force field with the incoming stream working radially inwardly against the centrifugal forces generated by the rotor and eventually being transformed into an outwardly moving stream from which the particles have been eliminated and entraining primarily the heavier particles. The guide elements may be formed as openings or passages between blades of a stator-blade ring disposed adjacent the coarse separation chamber. The return conduit which connects the discharge side of the coarse separator to the intake side of the fine separator opens tangentially into the classification chamber of the latter. In this fine separator, the particles entrained in the classifying air are in large measure cast by centrifugal force against the walls of the housing and are guided to the discharge gate. Only the finest particles therefore remain in the air which is fed back from this separator. According to another feature of the invention, the inlet conduit opens into the coarse separator at a housing portion which is rotationally symmetrical and coaxial withthe separating chamber. Advantageously, this chamber is frustoconically divergent in the direction of the separating chamber to a diameter equal to that of the separating chamber. Because of this rotationally symmetrical configuration of the inlet chamber, it is possible to provide in the latter distribution blades or fans which rotate about the axis of the rotor and serve to distribute the particulate or comminuted material uniformly over the circumference of the chamber. It prevents buildup of the comminuted material and enables the guide surfaces to operate more effectively and more uniformly.

It has also been found advantageous to provide means for accommodating the apparatus to different particle-size ranges and particles of different characteristics. To this end, the drive motor of at least the coarse-particle rotor is of adjustable speed and appropriate adjusting means is provided for the same. Of course, the rotor of the fine separator may also be controlled although this is not essential. In place of such motor control but preferably in conjunction therewith, one or both of the conduits connecting the two units are provided with a flap damper or other adjustable flow-control member which also serves to regulate the particle-size fraction connected in each chamber.

The problem of particle penetration into the fan space and especially the overflow of collected particles in the coarse-fraction collector (although the fine fraction collector is also faced with this problem), asecondary flow or air is provided to constitute an air gate blocking the transfer of solids across a partition separating the collection chamber from the blower chamber. The excess air supplied by an external source of compression air may be vented to the atmosphere via a filter sack.

It has also been pointed out that the present system operates in accordance with principles of both centrifugal and pneumatic classification, in that the rotors of the units may be axial intake, radial outflow blowers provided with cylindrical cages of rods or bars against which the air stream containing the particles is directed. The bars constitute centrifugal fans in their own right and cast light particles outwardly while deflecting heavier particles from the air stream or projecting these particles against the chamber walls, thereby permitting the particles to cascade to the collecting gates at the bottoms of the respective chambers. In this system, no extensive column or chamber of rising air is required. However, since the air circulating through the system gradually accumulates fine particles which are not collected in either unit, it is preferred to provide an auxiliary channel to which a portion of the air circulated from the fine separator to the coarse separator is diverted, this channel being provided with a fine-particle or dust filter for removing a fraction of the dust content of the recirculated air, thereby preventing excessive accumulation.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a front-elevational view of the airclassification apparatus according to the present invention;

FIG. 2 is a cross-section taken generally along the line II II of FIG. 1;

FIG. 3 is a cross-sectional view taken generally along the line III III of FIG. 1;

FIG. 4 is a cross-sectional view taken along the line IV IV of FIG. 1; and

FIG. 5 is a diagrammatic view similar to FIG. 1, but illustrating another embodiment of the present invention.

SPECIFIC DESCRIPTION As can best be seen from FIG. 1, the apparatus of the present invention comprises a fine-fraction separator l, and a coarse-fraction separator 2 mounted upon respective pedestals 28 and 29 and of generally cylindrical configuration. Each of the separators comprises a bar or blade-cage rotor 3 or 4 having axes A and B, respectively, which are parallel and transversely spaced.

At one axial end of each of these rotors, there is provided an axial-intake, radial outflow fan 5 or 6 which is driven by respective transmissions 30 and 31. As has been illustrated diagrammatically for the coarsefraction separator 2 in FIG. 3, the fan 6 and the respective cage 4 have a support sleeve 32 fixed by cones 33 and 34 to a shaft 35 joumaled in a pair of blocks 36 and 37 outwardly of the housing of the unit. At one end of this shaft 35, a pulley 38 is provided to constitute part of the transmission system. A belt 39 extends around this pulley and a pulley 40 which is driven by a motor 41 through a mechanical bevel gear transmission 42. A speed control 43 is provided for the motor 41 to regulate the particle size of the fraction collected in the coarse-particle separator. The blower and cage of the fine-fraction separator is similarly mounted and driven.

Each of the rotors 3, 5 and 4, 6 is received in a classifying chamber 7 or 8, respectively, with adjacent blower or fan chambers 9 and (see FIGS. 3 and 4). The chamber 7, 9 and 8, 10 are separated from one another by partitions 11, reaching inwardly to positions close to the respective rotors which are provided with circular hubs 13 so that the gaps 12 between the hubs and the confronting partitions form air locks or seals as represented at 12.

A supply conduit 14 (FIG. 1) connects the discharge side (i.e. the periphery of fan chamber 9) of the finefraction separator with the intake side 2 of the coarse fraction separator. The intake side of this separator is formed by a rotationally symmetrical frustoconical portion 16 at the axial end of the coarse-fraction separator 2 remote from the fan compartment 10. The frustoconical portion 16 widens in the direction of the separating chamber 8 until it assumes the diameter thereof. As can best be seen in FIG. 1, the duct 14 extends tangentially outwardly of the blower compartment 9 of the fineparticle separator and extends tangentially into the frustoconical portion 16 of the coarse-particle separator. A return conduit 15 extends tangentially from the blower chamber 10 of the coarse-particle separator and opens tangentially into the separating chamber 7 of the fine-particle classifier.

In the supply conduit 14, there is provided an inlet 17 for the comminuted material to be classified. This inlet is constituted as a lock of substantially conventional construction. For example, a cylindrical seal is flanged at 51 to an upstanding net 52 of the conduit 14 and communicates with the latter while being open at a flange 53 to be connected to a hoper or the like delivering the comminuted material. Within the cylinder 50, a semicylindrical closure member 54 is angularly displaceable as represented by arrow C from the position illustrated in which communication with the atmosphere is prevented by the comminuted material can fill the trough formed by member 54, and a position in which the trough opens downwardly to dump the comminuted material but nevertheless prevents communication with the atmosphere.

As best seen in FIGS. 3 and 4, the chambers 7 and 8 are each provided with a respective discharge gate 18 or 19 of the seal type described with respect to the gate 51 54 for discharging the fine and coarse particle fractions respectively. Between the fine-particle blower chamber 9 and the inlet 17, there is provided a bypass channel 20 (FIG. 2) in which a dust filter 21 is disposed to recover a portion of the dust from the recirculated air as previously described. 1

The coarse-fraction separator 2 is provided, ahead of the frustoconical housing 16 and within the latter, with a distributing-blade arrangement 22 (with a number of angularly equispaced blades) mounted upon the sleeve 32 for ensuring a uniform distribution of the comminuted material over the entire circumference of chamber 8. A bevel 45 ahead of the rotor 4 co-operates with guide elements 23 to ensure a direction of the comminuted material against the rotor 4 which is between the radial and the axial and, therefore, is generally spiral. In other words, particles are cast against the bars of the rotor in an off-radial direction somewhat transverse to the bars. As can best be seen from FIGS. 2 or 3, these guide elements are blades or fans formed on a blade ring mounted in the housing 8. In the return duct 15, an adjustable flap 24 is provided to control the circulated air stream.

The mixture of comminuted material with particles of different sizes ranges is introduced into the inlet 17 and is entrained by the air stream through duct 14 into the housing portion 16 from which it is spread circumferentially and is deflected around the bevel 45 and inwardly against the rotor 4 via the blades 23. The largeparticle fraction is cast outwardly and passes along the wall of the chamber 8 to the discharge duct 25 from which it is removed via the seal gate 19. Because of the relatively small diameter of the rotor 4 and the relatively high air speed, the fine particles continue to be entrained with the air stream and are passed through the duct to enter the chamber 7 tangentially. Again the particles encounter the rotor 3 and are cast against the wall of the chamber 7 to accumulate and be discharged via a hopper 26 and the gate 18. The remaining particles in the form of a fine dust are carried with the air stream through duct 14 while a part of this returning air is freed from particles in filter 21.

In FIG. 5, there is shown a modified system in which the coarse-fraction separator 102 has its rotor driven in the counterclockwise sense (arrow D) while the rotor of the fine-particle separator 101 is rotated in the clockwise sense (arrow E). The inlet duct 114 connects the fan chamber of the separator 101 with thefrustoconical portion 116 of separator 102 and is provided with an inlet gate 117 as previously described. The duct 1 15, however, connects the upper side of the fan chamber of separator 102 to the underside of the separator 101. Consequently, while the system of FIG. 1 provides an overpath for the classifying air, the system of FIG. 5 provides a path having the configuration of a figure 8.

I claim:

1. An apparatus for the classification of a comminuted material into a plurality of particle-size fractions, comprising:

at least two separators including a coarse-particle separator and a fine-particle separator, each of said separators being provided with:

a respective housing spaced from the other separator and defining a respective separating chamber,

a respective rotor means for rejecting a portion of said comminuted material delivered thereto journaled in each of said separating chambers,

a respective blower in each housing for inducing air flow through the respective rotor and chamber,

a respective discharge fitting on each housing for removal of respective particle-size fractions therefrom, and

a respective air intake and air outlet on each housing communicating with the respective chamber;

and 1 respective conduits connecting the outlet of said fineparticle separator with the intake of said coarseparticle separator and for connecting the outlet of said coarse particle separator with the intake 'of said fine-particle separator to form a closed air circuit between said separator, said outlets opening generally tangentially away from the respective chambers, the intake of said coarse-particle separator terminating in guide means for directing material against said rotor'means in an off-radial direction spaced around the rotor means thereof and the intake of said fine-particle separator opening generally tangentially into the chamber thereof.

2. The apparatus defined in claim 1 wherein said guide means form part of the fixed ring of guide blades and said housing of said coarse-particle separator is provided with a spiral portion ahead of said blades.

3. The apparatus defined in claim 1 wherein said chamber and said blower of said coarse-particle separator are oriented horizontally, said intake housing portion having a broad base turned toward the chamber thereof and provided with a rotary blade distributor.

4. The apparatus defined in claim 1, further comprising a gate communicating with the conduit connecting the outlet of said fine-particle separator with the intake of said coarse-particle separator.

5. The apparatus defined in claim 1 wherein said conduits and said separators are so constructed and arranged to provide a figure-8 pattern for the air flow between said separators.

6. The apparatus defined in claim 1 wherein said conduits and said separators are so constructed and arranged to provide an oval pattern for the airflow be tween said separators.

7. The apparatus defined in claim 1 wherein said intake of said coarse-particle separator comprises a rotationally symmetrical housing coaxial with the respective rotor means, said apparatus further comprising a distributing blade rotatable in said housing for spreading the comminuted material circumferentially about the chamber of said coarse-particle separator.

8. The apparatus defined in claim 7 wherein said housing frustoconically diverges in the direction of said chamber of said coarse-particle separator to the diameter thereof.

9. The apparatus defined in claim l, further comprising means for varying the speed'of at least one of said rotor means. 1

10. The apparatus defined in claim 1', further comprising valve means in at least one of said conduits for controlling the air flow along said circuit.

11. The apparatus definedin claim 1 wherein at least one of said separators is provided with a partition subdividing the interior thereof into a blower chamber and a rotor chamber, said apparatus further comprising means forming an air seal between said partition of said rotor means for preventing the passage of comminuted material from said rotor chamber to said blower chamber except as entrained by the air traversing the blower.

12. The apparatus defined in claim 1, further comprising bypass means between the outlet of said fineparticle' separator and the intake of said coarse-particle separator and including a filter for removing residual dust from the air delivered to said coarse-particle sepai i i i 

1. An apparatus for the classification of a comminuted material into a plurality of particle-size fractions, comprising: at least two separators including a coarse-particle separator and a fine-particle separator, each of said separators being provided with: a respective housing spaced from the other separator and defining a respective separating chamber, a respective rotor means for rejecting a portion of said comminuted material delivered thereto journaled in each of said separating chambers, a respective blower in each housing for inducing air flow through the respective rotor and chamber, a respective discharge fitting on each housing for removal of respective particle-size fractions therefrom, and a respective air intake and air outlet on each housing communicating with the respective chamber; and respective conduits connecting the outlet of said fine-particle separator with the intake of said coarse-particle separator and for connecting the outlet of said coarse-particle separator with the intake of said fine-particle separator to form a closed air circuit between said separator, said outlets opening generally tangentially away from the respective chambers, the intake of said coarse-particle separator terminating in guide means for directing material against said rotor means in an off-radial direction spaced around the rotor means thereof and the intake of said fine-particle separator opening generally tangentially into the chamber thereof.
 2. The apparatus defined in claim 1 wherein said guide means form part of the fixed ring of guide blades and said housing of said coarse-particle separator is provided with a spiral portion ahead of said blades.
 3. The apparatus defined in claim 1 wherein said chamber and said blower of said coarse-particle separator are oriented horizontally, said intake housing portion having a broad base turned toward the chamber thereof and provided with a rotary blade distributor.
 4. The apparatus defined in claim 1, further comprising a gate communicating with the conduit connecting the outlet of said fine-particle separator with the intake of said coarse-particle separator.
 5. The apparatus defined in claim 1 wherein said conduits and said separators are so constructed and arranged to provide a figure-8 pattern for the air flow between said separators.
 6. The apparatus defined in claim 1 wherein said conduits and said separators are so constructed and arranged to provide an oval pattern for the airflow between said separators.
 7. The apparatus defined in claim 1 wherein said intake of said coarse-particle separator comprises a rotationally symmetrical housing coaxial with the respective rotor means, said apparatus further comprising a distributing blade rotatable in said housing for spreading the comminuted material circumferentially about the chamber of said coarse-particle separator.
 8. The apparatus defined in claim 7 wherein said housing frustoconically diverges in the direction of said chamber of said coarse-particle separator to the diameter thereof.
 9. The apparatus defined in claim 1, further comprising means for varying the speed of at least one of said rotor means.
 10. The apparatus defined in claim 1, further comprising valve means in at least one of said conduits for controlling the air flow along said circuit.
 11. The apparatus defined in claim 1 wherein at least one of said separators is provided with a partition subdividing the interior thereof into a blower chamber and a rotor chamber, said apparatus further comprising means forming an air seal between said partition of said rotor means for preventing the passage of comminuted material from said rotor chamber to said blower chamber except as entrained by the air traversing the blower.
 12. The apparatus defined in claim 1, further comprising bypass means between the outlet of said fine-particle separator and the intake of said coarse-particle separator and including a filter for removing residual dust from the air delivered to said coarse-particle separator. 